The facet joints, or zygapophyseal joints, of the spine are located at two symmetrical locations at the posterior of the vertebral column. Each facet joint consists of two overlapping bony protrusions, the superior articular process of one vertebrae and the inferior articular process of the neighboring vertebrae. FIG. 18 illustrates two vertebrae as they mate at the facet or zygapophyseal joints.
In certain cases of degeneration of the spinal disk, instability of vertebral segments, arthritis of the facet joint, or trauma, partial or complete immobilization of one or more facet joints is desirable. Traditionally, immobilization is accomplished by anchoring orthopedic hardware into the vertebral bodies of adjacent segments, often through the pedicle, and interposing a plate or rod between the vertebrae to limit motion. Additionally, interbody devices are often placed into the disc space through a variety of techniques to further limit motion and promote bony fusion between adjacent vertebrae. However, for a number of reasons, it would be advantageous to eliminate motion and improve stability between two or more vertebrae by directly fastening one or both of the facet joints together. From a surgical perspective, the facet joint is easily accessible, thus reducing operative time, decreasing blood loss, decreasing incision size, reducing incidence of reoperation, and decreasing the risk of potential deleterious effects on nearby anatomic structures, including spinal nerve roots and the spinal cord itself. Further, fixation at the facet joints is more biomechanically desirable because the center of rotation of the lumbar spine for flexion and extension is located nearest to the facet joints. Thus, placing an immobilization device at or through the facet joint decreases the torque transmitted through the device, which in turn may prevent loosening or premature device failure.
In order to provide effective fixation of the facet joint, a few challenges must be overcome. Most importantly, a fixation device must create compression between the two articular processes. The compression, which causes or enhances immobilization of the joint by encouraging stability throughout the joint, must be maintained for a significant period of time. Additionally, loosening of the device must be prevented. Because the facet joint is generally a mobile joint, forces will continue to be transmitted through the joint after the insertion of an immobilization device. Without a specific way to prevent loosening of the device, loosening will likely occur as the result of micromotion. Once a device has loosened, the device will often begin to protrude from or back-out of the bone, causing significant discomfort, damage to the joint, or danger to surrounding tissues.
Other devices, such as various types of bone screws, have been offered as ways to fasten the facet joints together. However, each previously proposed fixation device has suffered significant shortcomings. For example, a standard fully threaded bone screw may be sufficient for merely adjoining two surfaces. However, a fully threaded screw is not capable of creating any significant amount of compression between two bone surfaces. Any compression generated between surfaces is limited to the compressive forces generated by the screw threads themselves. Further, there is currently no way to effectively prevent a bone screw from loosening over time. When a screw is over-tightened and threads are stripped within the bone, or when threads strip over time as a result of micromotion, the compressive force between the facet joint surfaces will be lost and loosening will likely occur.
To prevent loosening, still other bone screws are designed such that a portion of the screw expands within the bone after the device is implanted. However, the hoop stress generated by expansion of the device within a bone makes this device ill-suited for use in the relatively small bones of the facet joint.
In attempt to create compression and prevent loosening, nut-and-bolt type assemblies have been offered as another method for immobilization of the facet joint. Using this type of assembly, a screw or “bolt” is passed through the facet joint and a nut with mating threads is placed around the screw on the back side of the facet. This approach is successful in creating compression and likely at maintaining the compression over time. However, because the nut must be introduced to the back side of the facet joint, this approach mandates a procedure that is significantly more invasive than is otherwise required.
Finally, many devices currently available for fixation of the facet joint do not contain a central hollow and therefore are not equipped for use with a guide wire, as is known in the art of orthopedic devices. Without a guide wire, placement of the device within the bones is less efficient and accuracy is more difficult. Further, small devices, such as bone screws, can be dropped and even lost within the soft tissue surrounding the site of insertion.
Because of the shortcomings associated with the currently available facet immobilization devices, physicians have largely been hesitant to attempt immobilization of the facet joint, despite the significant biomechanical and surgical benefits of doing so.
As such, there is a considerable need for a facet fixation device that can be easily and effectively inserted through a small incision and extend through the inferior and superior articular processes in order to create active compression across the facet joint and limit loosening over time.